It is previously known to prepare paper containing plastic particles in order to obtain a paper product having a light weight but with the service properties of heavier paper. For instance Swedish Patent 300,927 describes a process wherein expanded plastic beads are added to a fiber pulp before the pulp is dewatered over a wire screen, whereafter the paper is formed in a known manner. This method is not entirely satisfactory since there will be difficulties in achieving sufficient retention between the cellulose fibers and in achieving a good enough dispersion of the plastic beads in the pulp to obtain a uniform paper. Such paper will often exhibit a pronounced two-sidedness, since the polymer particles to a great extent will be present on the side of the paper that is furthest from the wire. Furthermore, the surface of the finished paper frequently contains bubbles or foam particles from the plastic, which cause disturbances if they are too large or if they come off the surface. To obtain acceptable paper by this known process some sort of surface sizing must be carried out, which increases the price of the finished paper.
Several methods have also been suggested for the preparation of laminated paper, wherein a middle-layer is made up from an expanded plastic. These methods utilize two or more finished paper sheets between which a layer of expandable or expanded plastic is laminated. According to one method a film of an expandable plastic is introduced beween the paper sheets and the laminate is then bonded by heating and calendering. It has also been suggested to prepare a laminate by introduction of expanded plastic beads between two paper sheets having an adhesive coating so that the finished laminate will have a layer of expanded plastic glued to the paper. Furthermore, it is known to introduce a water or solvent dispersion of an expandable plastic between two paper sheets and obtain a paper laminate by drying and heating. These lamination methods start with pre-fabricated paper sheets which are subjected to some kind of subsequent treatment, such as heating or drying, in order to obtain a paper containing plastic particles. This subsequent treatment naturally increases the production price and the obtained laminate will show two spaced apart paper layers bonded together by the plastic layer. In order to obtain a satisfactory z-strength, i.e. bonding between the three layers, the demands on process equipment and process control must be high.